Circuit substrate and method of manufacturing same

ABSTRACT

A circuit substrate capable of reducing and preventing deviations of circuit characteristics includes a relatively hard region and a relatively soft region. A main body of the circuit substrate includes a stack of a plurality of flexible sheets made of a flexible material and includes rigid regions and a flexible region, the flexible region being more easily deformable than the rigid regions. Wiring conductors are disposed in the main body and define circuitry. Reinforcing insulative films are disposed so as to cover the portions where the wiring conductors are not disposed in the rigid regions on the flexible sheets when seen in plan view from the z-axis direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit substrate and a method ofmanufacturing the same and, in particular, to a circuit substrateincluding a rigid region and a flexible region and a method ofmanufacturing the same.

2. Description of the Related Art

One known example of a traditional circuit substrate is a wiringsubstrate described in Japanese Unexamined Patent ApplicationPublication No. 2006-339186. The wiring substrate described in JapaneseUnexamined Patent Application Publication No. 2006-339186 includes aflexible section and a rigid section disposed to be contiguous to theflexible section. The flexible section includes a flexible substrate inwhich wiring patterns are stacked such that an insulative resin layer isdisposed therebetween. The rigid section includes a flexible substrateformed integrally with the flexible section. The wiring density of thewiring patterns in the rigid section is higher than that in the flexiblesection. Thus, the rigid section has a higher hardness than that of theflexible section.

However, for the wiring substrate described in Japanese UnexaminedPatent Application Publication No. 2006-339186, the stray capacitancemay increase, and the circuit characteristics may deviate from a desiredvalue. More specifically, for the wiring substrate described in JapaneseUnexamined Patent Application Publication No. 2006-339186, to have ahigher wiring density of the wiring patterns in the rigid section thanthat in the flexible section, a redundant wiring pattern is disposed inthe rigid section. The redundant wiring pattern faces other wiringpatterns and forms unnecessary stray capacitance. As a result, thecircuit characteristics in the wiring substrate described in JapaneseUnexamined Patent Application Publication No. 2006-339186 deviate from adesired value.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a circuitsubstrate and a method of manufacturing the circuit substrate, thecircuit substrate having circuit characteristics in which the occurrenceof deviations can be reduced and including a relatively hard region anda relatively soft region.

A circuit substrate according to a preferred embodiment of the presentinvention includes a main body including a stack of a plurality of firstinsulator layers made of a flexible material, the main body including arigid region and a flexible region, the flexible region being moreeasily deformable than the rigid region, a conductive layer disposed inthe main body and forming circuitry, and a second insulator layerdisposed so as to cover at least a portion of an area where theconductive layer is not disposed in the rigid region on at least one ofthe first insulator layers when seen in plan view from a stackingdirection.

A method of manufacturing a circuit substrate according to anotherpreferred embodiment of the present invention includes a step ofpreparing a plurality of first insulator layers on which circuitry madeof a conductive layer is formed, the plurality of first insulator layersbeing made of a flexible material, a step of forming a second insulatorlayer so as to cover at least a portion of an area where the conductivelayer is not disposed on at least one of the first insulator layers whenseen in plan view from a stacking direction, and a step of stacking andpress-bonding the plurality of first insulator layers.

According to various preferred embodiments of the present invention, acircuit substrate that has circuit characteristics in which theoccurrence of deviations can be reduced and that includes a relativelyhard region and a relatively soft region is obtainable.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external perspective view of a circuit substrate accordingto a preferred embodiment of the present invention.

FIG. 2 is an exploded perspective view of the circuit substrateillustrated in FIG. 1.

FIGS. 3A and 3B are perspective views of a flexible sheet in the circuitsubstrate in its manufacturing process.

FIG. 4 is a cross-sectional configuration view of the circuit substrateillustrated in FIG. 2 taken along the line A-A.

FIG. 5 is an exploded perspective view of a circuit substrate accordingto a first variation of a preferred embodiment of the present invention.

FIG. 6 is an exploded perspective view of a circuit substrate accordingto a second variation of a preferred embodiment of the presentinvention.

FIG. 7 is a cross-sectional configuration view of a circuit substrateaccording to a third variation of a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A circuit substrate and a method of manufacturing the same according topreferred embodiments of the present invention are described below withreference to the drawings.

A configuration of a circuit substrate according to a preferredembodiment of the present invention is described below with reference tothe drawings. FIG. 1 is an external perspective view of a circuitsubstrate 10 according to a preferred embodiment of the presentinvention. FIG. 2 is an exploded perspective view of the circuitsubstrate 10 illustrated in FIG. 1. FIGS. 3A and 3B are perspectiveviews of a flexible sheet 26 a in the circuit substrate 10 in itsmanufacturing process. FIG. 3A illustrates the back side of the flexiblesheet 26 a, and FIG. 3B illustrates the front side of the flexible sheet26 a in a state where resist films 20 a and 24 a are not formed. FIG. 4is a cross-sectional configuration view of the circuit substrate 10illustrated in FIG. 2 taken along the line A-A. In FIGS. 1 to 4, thestacking direction of the circuit substrate 10 is defined as the z-axisdirection, the longitudinal direction of a line section 16 in thecircuit substrate 10 is defined as the x-axis direction, and thedirection to which the x-axis direction and the z-axis direction areorthogonal is defined as the y-axis direction. The front side of each ofthe circuit substrate 10 and the flexible sheets 26 indicates a surfacepositioned on the positive-direction side in the z-axis direction, andthe back side of each of the circuit substrate 10 and the flexible sheet26 indicates a surface positioned on the negative-direction side in thez-axis direction.

As illustrated in FIG. 1, the circuit substrate 10 includes a main body11 including substrate sections 12 and 14 and the line section 16. Asillustrated in FIG. 2, the main body 11 includes a stack of a pluralityof (for example, preferably four in FIG. 2) flexible sheets (insulatorlayers) 26 (26 a to 26 d) made of a flexible material (e.g.,thermoplastic resin, such as liquid crystal polymer or polyimide). Eachof the flexible sheets 26 preferably has a Young's modulus ofapproximately 2 GPa to 20 GPa, for example.

The substrate section 12 is substantially rectangular and includes, onthe front side, an implementation surface on which a plurality of chipcomponents 50 and an integrated circuit 52 are implemented. Thesubstrate section 14 has the shape of a substantially rectangle smallerthan the substrate section 12 and includes, on the front side, animplementation surface on which a connector 54 is implemented. Thesubstrate sections 12 and 14 are not prone to deforming (bending) toallow the chip components 50, integrated circuit 52, and connector 54 tobe stably implemented therein. Hereinafter, the substrate sections 12and 14 are also referred to as rigid regions R1 and R2, respectively.The line section 16 connects the substrate sections 12 and 14 together.The circuit substrate 10 is used in a state where the line section 16 iscurved in a substantially U shape. The line section 16 is easilydeformable (bendable). Hereinafter, the line section 16 is also referredto as a flexible region F1.

First, the substrate section 12 (rigid region R1) is described. Asillustrated in FIG. 2, the substrate section 12 includes a stack ofsubstrate-section sheets 27 a to 27 d of the flexible sheets 26 a to 26d. As illustrated in FIGS. 1 to 3, the substrate section 12 includes theresist film 20 a, reinforcing insulative films 20 b to 20 d, lands 28,wiring conductors 30 (30 b, 30 c), a ground conductor 37, and via-holeconductors b1 to b3 and b21 to b26. In FIGS. 1 to 3, reference numeralsare provided to only representative ones of the lands 28, the wiringconductors 30, and the via-hole conductors b1 to b3 to preventcomplication in the drawings.

Each of the substrate-section sheets 27 a to 27 d of the flexible sheets26 a to 26 d corresponds to a first insulator layer according to apreferred embodiment of the present invention, and each of thereinforcing insulative films 20 b to 20 d corresponds to a secondinsulator layer according to a preferred embodiment of the presentinvention. The same applies to variations described below.

The lands 28 are disposed in the main body 11 and, preferably aredefined by a conductive layer disposed on the front side of thesubstrate-section sheet 27 a, as illustrated in FIG. 2. The chipcomponents 50 and the integrated circuit 52 are implemented on the lands28 preferably by soldering, for example, as illustrated in FIG. 1.

As illustrated in FIG. 3A, the via-hole conductors b1 penetrate throughthe substrate-section sheet 27 a along the z-axis direction. Thevia-hole conductors b1 are connected to the lands 28.

The wiring conductors 30 b are disposed in the main body 11 and,preferably are a conductive layer disposed on the front side of thesubstrate-section sheet 27 b, as illustrated in FIG. 2. The via-holeconductors b2 penetrate through the substrate-section sheet 27 b alongthe z-axis direction, as illustrated in FIG. 2. The via-hole conductorsb2 are connected to the via-hole conductors b1. As illustrated in FIG.2, the via-hole conductors b21 to b23 penetrate through thesubstrate-section sheet 27 b along the z-axis direction. The via-holeconductors b21 to b23 are connected to the wiring conductors 30 b.

The wiring conductors 30 c are disposed in the main body 11 and,preferably are a conductive layer disposed on the front side of thesubstrate-section sheet 27 c, as illustrated in FIG. 2. The via-holeconductors b3 penetrate through the substrate-section sheet 27 c alongthe z-axis direction, as illustrated in FIG. 2. Each of the via-holeconductors b3 is connected to one of the via-hole conductors b2. Asillustrated in FIG. 2, the via-hole conductors b24 to b26 penetratethrough the substrate-section sheet 27 c along the z-axis direction. Thevia-hole conductors b24 to b26 are connected to the via-hole conductorsb21 to b23, respectively.

The ground conductor 37 is disposed in the main body 11 and, preferablyis a single film electrode having the shape of a substantially rectanglearranged to cover the front side of the substrate-section sheet 27 d. Asillustrated in FIG. 2, not all of the substrate-section sheet 27 d iscovered by the ground conductor 37; the ground conductor 37 is notdisposed in the vicinity of the outer regions of the substrate-sectionsheet 27 d. The ground conductor 37 is grounded and thus maintained at aground potential. The ground conductor 37 is connected to the via-holeconductors b3 and b24 to b26. As described above, the substrate-sectionsheets 27 a to 27 d are stacked, and thus the wiring conductors 30 b and30 c, the ground conductor 37, and the via-hole conductors b1 to b3 andb21 to b26 are connected to each other and define circuitry.

The resist film 20 a is disposed so as to cover the front side of thesubstrate-section sheet 27 a and is an insulative film to protect thesubstrate-section sheet 27 a. The resist film 20 a is not disposed onthe lands 28. The resist film 20 a is a solder resist film to define anarea in which solder is to be applied on the lands 28.

The reinforcing insulative film 20 b is disposed in the main body 11and, preferably is an insulative film that covers the portion where thewiring conductors 30 b and the via-hole conductors b2 and b21 to b23 arenot disposed on the front side of the substrate-section sheet 27 b (inthe rigid region R1) when seen in plan view from the z-axis direction,as illustrated in FIG. 2. The thickness of the reinforcing insulativefilm 20 b is equal to or less than the thickness of each of the wiringconductors 30 b. In the present preferred embodiment, the thickness ofthe reinforcing insulative film 20 b is equal to the thickness of thewiring conductor 30 b. The reinforcing insulative film 20 b is made of amaterial that is harder than the material of the substrate-section sheet27 b and can be produced by application of thermosetting resin (e.g.,epoxy resin), for example. The reinforcing insulative film 20 bpreferably has a Young's modulus of approximately 12 GPa to 30 GPa, forexample.

The reinforcing insulative film 20 c is disposed in the main body 11and, preferably is an insulative film that covers the portion where thewiring conductors 30 c and the via-hole conductors b3 and b24 to b26 arenot disposed on the front side of the substrate-section sheet 27 c (inthe rigid region R1) when seen in plan view from the z-axis direction,as illustrated in FIGS. 2 and 4. The thickness of the reinforcinginsulative film 20 c is equal to or less than the thickness of each ofthe wiring conductors 30 c, as illustrated in FIG. 4. In the presentpreferred embodiment, the thickness of the reinforcing insulative film20 c preferably is equal to the thickness of the wiring conductor 30 c.The reinforcing insulative film 20 c is made of a material harder thanthe material of the substrate-section sheet 27 c and can be produced byapplication of thermosetting resin (e.g., epoxy resin), for example. Thereinforcing insulative film 20 c preferably has a Young's modulus ofapproximately 12 GPa to 30 GPa, for example.

The reinforcing insulative film 20 d is disposed in the main body 11and, preferably is an insulative film that covers the portion where theground conductor 37 is not disposed on the front side of thesubstrate-section sheet 27 d (in the rigid region R1) when seen in planview from the z-axis direction, as illustrated in FIG. 2. The thicknessof the reinforcing insulative film 20 d is equal to or less than thethickness of the ground conductor 37. In the present preferredembodiment, the thickness of the reinforcing insulative film 20 d isequal to the thickness of the ground conductor 37. The reinforcinginsulative film 20 d preferably is made of a material that is harderthan the material of the substrate-section sheet 27 d and can beproduced by application of thermosetting resin (e.g., epoxy resin), forexample. The reinforcing insulative film 20 d preferably has a Young'smodulus of approximately 12 GPa to 30 GPa, for example.

It is preferable that each of these reinforcing insulative films may bedisposed in the entire portion where the ground conductor is notdisposed on the substrate-section sheet in the substrate section 12(rigid region R1), as described above. Alternatively, they may bedisposed in a portion of the portion where the ground conductor is notdisposed. Alternatively, only one of the plurality of substrate-sectionsheets being stacked may be overlaid with the reinforcing insulativefilm.

In the case where the reinforcing insulative film is disposed in aportion of an area where the ground conductor is not disposed on thesubstrate-section sheet, the reinforcing insulative film may preferablybe disposed on the substrate-section sheet in the vicinity of the borderbetween the substrate section 12 (rigid region R1) and the line section(flexible region F1). The reinforcing insulative film may preferably bedisposed on the substrate-section sheet in a fixation portion fixed on acasing or mother board in the rigid region R1. In addition, in the casewhere a component is mounted on or incorporated in the rigid region R1,the reinforcing insulative film may preferably be disposed on thesubstrate-section sheet adjacent to the mounting side, or on each of thesubstrate-section sheets above and below the component incorporated, inthe region that overlaps the component when the circuit substrate 10 isseen in plan view from the stacking direction.

Next, the substrate section 14 (rigid region R2) is described. Asillustrated in FIG. 2, the substrate section 14 includes a stack ofsubstrate-section sheets 29 a to 29 d of the flexible sheets 26 a to 26d. As illustrated in FIGS. 1 to 3, the substrate section 14 includes theresist film 24 a, reinforcing insulative films 24 b to 24 d, lands 35,wiring conductors 36 (36 b, 36 c), a ground conductor 40, and via-holeconductors b11, b12, and b31 to b36. In FIGS. 1 to 3, reference numeralsare provided to only representative ones of the lands 35, the wiringconductors 36, and the via-hole conductors b11 and b12 to preventcomplication in the drawings.

The lands 35 are disposed in the main body 11 and, preferably areconductive layers disposed on the front side of the substrate-sectionsheet 29 a, as illustrated in FIG. 2. The connector 54 is implemented onthe lands 35 preferably by soldering, as illustrated in FIG. 1, forexample.

As illustrated in FIG. 3A, the via-hole conductors b11 penetrate throughthe substrate-section sheet 29 a along the z-axis direction. Thevia-hole conductors b11 are connected to the lands 35.

The wiring conductors 36 b are disposed in the main body 11 and,preferably are a conductive layer disposed on the front side of thesubstrate-section sheet 29 b, as illustrated in FIG. 2. The via-holeconductors b12 penetrate through the substrate-section sheet 29 b alongthe z-axis direction, as illustrated in FIG. 2. The via-hole conductorsb12 are connected to the via-hole conductors b11. As illustrated in FIG.2, the via-hole conductors b31 to b33 penetrate through thesubstrate-section sheet 29 b along the z-axis direction. The via-holeconductors b31 to b33 are connected to the wiring conductors 36 b.

The wiring conductors 36 c are disposed in the main body 11 and,preferably are a conductive layer disposed on the front side of thesubstrate-section sheet 29 c, as illustrated in FIG. 2. The wiringconductors 36 c are connected to the via-hole conductors b12. Asillustrated in FIG. 2, the via-hole conductors b34 to b36 penetratethrough the substrate-section sheet 29 c along the z-axis direction. Thevia-hole conductors b34 to b36 are connected to the via-hole conductorsb31 to b33, respectively.

The ground conductor 40 is disposed in the main body 11 and, preferablyis a single film electrode having the shape of a substantially rectanglearranged so as to cover the front side of the substrate-section sheet 29d. As illustrated in FIG. 2, not all of the substrate-section sheet 29 dis covered by the ground conductor 40; the ground conductor 40 is notdisposed in the vicinity of the outer regions of the substrate-sectionsheet 29 d. The ground conductor 40 is grounded and thus maintained at aground potential. The ground conductor 40 is connected to the via-holeconductors b34 to b36. As described above, the substrate-section sheets29 a to 29 d are stacked, and thus the wiring conductors 36 b and 36 c,the ground conductor 40, and the via-hole conductors b11, b12, and b31to b36 are connected to each other and define circuitry.

The resist film 24 a is disposed so as to cover the front side of thesubstrate-section sheet 29 a and is an insulative film to protect thesubstrate-section sheet 29 a. The resist film 24 a is not disposed onthe lands 35. The resist film 24 a is a solder resist to define an areain which solder is to be applied on the lands 35.

The reinforcing insulative film 24 b is disposed in the main body 11and, preferably is an insulative film that covers the portion where thewiring conductors 36 b and the via-hole conductors b12 and b31 to b33are not disposed on the front side of the substrate-section sheet 29 b(in the rigid region R2) when seen in plan view from the z-axisdirection, as illustrated in FIG. 2. The thickness of the reinforcinginsulative film 24 b is equal to or less than the thickness of each ofthe wiring conductors 36 b. In the present preferred embodiment, thethickness of the reinforcing insulative film 24 b is equal to thethickness of the wiring conductor 36 b. The reinforcing insulative film24 b is preferably made of a material that is harder than the materialof the substrate-section sheet 29 b and can be produced by applicationof thermosetting resin (e.g., epoxy resin), for example. The reinforcinginsulative film 24 b has a Young's modulus of approximately 12 GPa to 30GPa, for example.

The reinforcing insulative film 24 c is disposed in the main body 11and, preferably is an insulative film that covers the portion where thewiring conductors 36 c and the via-hole conductors b34 to b36 are notdisposed on the front side of the substrate-section sheet 29 c (in therigid region R2) when seen in plan view from the z-axis direction, asillustrated in FIG. 2. The thickness of the reinforcing insulative film24 c is equal to or less than the thickness of each of the wiringconductors 36 c. In the present preferred embodiment, the thickness ofthe reinforcing insulative film 24 c is preferably equal to thethickness of the wiring conductor 36 c. The reinforcing insulative film24 c is preferably made of a material that is harder than the materialof the substrate-section sheet 29 c and can be produced by applicationof thermosetting resin (e.g., epoxy resin), for example. The reinforcinginsulative film 24 c has a Young's modulus of approximately 12 GPa to 30GPa, for example.

The reinforcing insulative film 24 d is disposed in the main body 11and, preferably is an insulative film that covers the portion where theground conductor 40 is not disposed on the front side of thesubstrate-section sheet 29 d (in the rigid region R2) when seen in planview from the z-axis direction, as illustrated in FIG. 2. The thicknessof the reinforcing insulative film 24 d is equal to or less than thethickness of the ground conductor 40. In the present preferredembodiment, the thickness of the reinforcing insulative film 24 d ispreferably equal or substantially equal to the thickness of the groundconductor 40. The reinforcing insulative film 24 d preferably is made ofa material that is harder than the material of the substrate-sectionsheet 29 d and can be produced by application of thermosetting resin(e.g., epoxy resin), for example. The reinforcing insulative film 24 dpreferably has a Young's modulus of approximately 12 GPa to 30 GPa, forexample.

Next, the line section 16 (flexible region F1) is described. Asillustrated in FIG. 2, the line section 16 includes a stack ofline-section sheets 31 a to 31 d of the flexible sheets 26 a to 26 d. Asillustrated in FIGS. 1 and 2, the line section 16 includes ground lines32 (32 b, 32 d), 33 (33 b, 33 d), and 34 (34 b, 34 d) and signal lines42 c, 43 c, and 44 c.

Each of the signal lines 42 c, 43 c, and 44 c is disposed in the mainbody 11 and, preferably is disposed in the line section 16 and extendsbetween the substrate sections 12 and 14. As illustrated in FIG. 2, eachof the signal lines 42 c, 43 c, and 44 c preferably is a linearconductive layer disposed on the front side of the line-section sheet 31c. A signal of a high frequency (e.g., 800 MHz to 900 MHz) istransmitted to the signal lines 42 c, 43 c, and 44 c. As illustrated inFIG. 2, the signal lines 42 c, 43 c, and 44 c connect the wiringconductors 30 c and the wiring conductors 36 c. That is, the conductivelayer including the wiring conductors 30 c and 36 c and the signal lines42 c, 43 c, and 44 c extends across the border between the flexibleregion F1 and each of the rigid regions R1 and R2.

Each of the ground lines 32 b, 33 b, and 34 b is disposed in the mainbody 11 and, preferably is disposed in the line section 16 andpositioned on the positive-direction side in the z-axis direction withrespect to the signal lines 42 c, 43 c, and 44 c. As illustrated in FIG.2, each of the ground lines 32 b, 33 b, and 34 b is disposed on thefront side of the line-section sheet 31 b and connects the wiringconductor 30 b and the wiring conductor 36 b. That is, the conductivelayer including the wiring conductors 30 b and 36 b and the ground lines32 b, 33 b, and 34 b extends across the border between the flexibleregion F1 and each of the rigid regions R1 and R2. In addition, thewiring conductors 30 b are connected to the ground conductor 37 throughthe via-hole conductors b21 to b26. The wiring conductors 36 b areconnected to the ground conductor 40 through the via-hole conductors b31to b36. Accordingly, each of the ground lines 32 b, 33 b, and 34 b iselectrically connected to the ground conductor 37. Each of the groundlines 32 b, 33 b, and 34 b is electrically connected to the groundconductor 40.

As illustrated in FIG. 2, the ground lines 32 b, 33 b, and 34 bpreferably have line widths that are wider than those of the signallines 42 c, 43 c, and 44 c, respectively. Therefore when seen in planview from the z-axis direction, the signal lines 42 c, 43 c, and 44 c donot protrude from the ground lines 32 b, 33 b, and 34 b, respectively,and overlap the ground lines 32 b, 33 b, and 34 b, respectively.

The ground lines 32 d, 33 d, and 34 d are disposed in the line section16 and positioned on the negative-direction side in the z-axis directionwith respect to the signal lines 42 c, 43 c, and 44 c. Specifically, asillustrated in FIG. 2, each of the ground lines 32 d, 33 d, and 34 d isdisposed on the front side of the line-section sheet 31 d and connectsthe ground conductor 37 and the ground conductor 40. That is, theconductive layer including the ground conductors 37 and 40 and theground lines 32 d, 33 d, and 34 d extends across the border between theflexible region F1 and each of the rigid regions R1 and R2.

As illustrated in FIG. 2, the ground lines 32 d, 33 d, and 34 dpreferably have line widths that are wider than those of the signallines 42 c, 43 c, and 44 c, respectively. Therefore, when seen in planview from the z-axis direction, the signal lines 42 c, 43 c, and 44 c donot protrude from the ground lines 32 d, 33 d, and 34 d, respectively,and overlap the ground lines 32 d, 33 d, and 34 d, respectively.

As described above, the ground lines 32 b, 33 b, and 34 b, the signallines 42 c, 43 c, and 44 c, and the ground lines 32 d, 33 d, and 34 doverlap each other. Therefore, the ground line 32 b, the signal line 42c, and the ground line 32 d define a strip line structure. Similarly,the ground line 33 b, the signal line 43 c, and the ground line 33 ddefine a strip line structure. The ground line 34 b, the signal line 44c, and the ground line 34 d define a strip line structure. As a result,the impedance between the circuitry in the substrate section 12 and thecircuitry in the substrate section 14 is matched. Thus, in the main body11, the circuitry in the substrate section 12, the circuitry in thesubstrate section 14, and the strip lines in the line section 16 definea single circuit having matched impedance.

A non-limiting method of manufacturing the circuit substrate 10according to another preferred embodiment of the present invention isdescribed below with reference to the drawings. In the followingdescription, the case where one circuit substrate 10 is produced isdescribed as an example. In actuality, however, a plurality of circuitsubstrates 10 preferably are produced at one time by cutting a stack oflarge flexible sheets.

First, flexible sheets 26 made of thermoplastic resin, such as a liquidcrystal polymer or polyimide, and having copper foil with a thickness of5 μm to 50 μm formed over the front side thereof are prepared. Thethickness of each of the flexible sheets 26 is approximately 10 μm to150 μm. Next, each of the locations where the via-hole conductors b1 tob3, b11, b12, b21 to b26, and b31 to b36 are to be formed in theflexible sheets 26 a to 26 c (see FIGS. 2 and 3A) is radiated with alaser beam from the back side thereof, thus forming via holes therein.

Next, the lands 28 and 35 illustrated in FIG. 3B are formed on the frontside of the flexible sheet 26 a by a photolithography step.Specifically, a resist having the same shape as that of each of thelands 28 and 35 illustrated in FIG. 3B is printed on the copper foil ofthe flexible sheet 26 a. The copper foil in the portion that is notcovered by the resists is removed by etching performed on the copperfoil. After that, the resists are removed. In this way, the lands 28 and35 illustrated in FIG. 3B are formed on the front side of the flexiblesheet 26 a. Then the resist films 20 a and 24 a illustrated in FIGS. 1and 2 are formed by application of resin on the front side of theflexible sheet 26 a.

Next, the wiring conductors 30 b and 36 b and the ground lines 32 b, 33b, and 34 b illustrated in FIG. 2 are formed on the front side of theflexible sheet 26 b by a photolithography step. The wiring conductors 30c and 36 c and the signal lines 42 c, 43 c, and 44 c illustrated in FIG.2 are formed on the front side of the flexible sheet 26 c by aphotolithography step. The ground lines 32 d, 33 d, and 34 d and theground conductors 37 and 40 illustrated in FIG. 2 are formed on thefront side of the flexible sheet 26 d by a photolithography step. Thesephotolithography steps are substantially the same as thephotolithography step used in the formation of the lands 28 and 35, sothe description thereof is omitted.

Next, the via holes in the flexible sheets 26 a to 26 c are filled withconductive paste including an alloy of tin and silver as the principalcomponent, thus forming the via-hole conductors b1 to b3, b11, b12, b21to b26, and b31 to b36 illustrated in FIGS. 2 and 3A. Through theabove-described steps, the flexible sheets, 26 a to 26 d being made of aflexible material, and on which the circuitry is made are prepared. Thecircuitry is made up of the wiring conductors 30 b, 30 c, 36 b, and 36c, the ground conductors 37 and 40, the via-hole conductors b1 to b3,b11, b12, b21 to b26, b31 to b36, the ground lines 32 b, 33 b, 34 b, 32d, 33 d, and 34 d, and the signal lines 42 c, 43 c, and 44 c.

Next, the reinforcing insulative film 20 b is formed on thesubstrate-section sheet 27 b by application of resin so as to cover theportion in which the wiring conductors 30 b and the via-hole conductorsb2 and b21 to b23 are not disposed when seen in plan view from thez-axis direction. The reinforcing insulative film 24 b is formed on thesubstrate-section sheet 29 b by application of resin so as to cover theportion in which the wiring conductors 36 b and the via-hole conductorsb12 and b31 to b33 are not disposed when seen in plan view from thez-axis direction. The reinforcing insulative film 20 c is formed on thesubstrate-section sheet 27 c by application of resin so as to cover theportion in which the wiring conductors 30 c and the via-hole conductorsb3 and b24 to b26 are not disposed when seen in plan view from thez-axis direction. The reinforcing insulative film 24 c is formed on thesubstrate-section sheet 29 c by application of resin so as to cover theportion in which the wiring conductors 36 c and the via-hole conductorsb34 to b36 are not disposed when seen in plan view from the z-axisdirection. The reinforcing insulative film 20 d is formed on thesubstrate-section sheet 27 d by application of resin so as to cover theportion in which the ground conductor 37 is not disposed when seen inplan view from the z-axis direction. The reinforcing insulative film 24d is formed on the substrate-section sheet 29 d by application of resinso as to cover the portion in which the ground conductor 40 is notdisposed when seen in plan view from the z-axis direction. Theapplication of resin is made by printing of liquid thermosetting epoxyresin by screen printing, gravure printing, or other process. Thethickness of each of the reinforcing insulative films 20 b to 20 d and24 b to 24 d may preferably be about 5 μm to about 50 μm, for example.

Lastly, the flexible sheets 26 a to 26 d are stacked in this order. Theflexible sheets 26 a to 26 d are press-bonded by the application offorce from both sides in the z-axis direction and the application ofheat. This causes the unhardened reinforcing insulative films 20 b to 20d and 24 b to 24 d to be hardened and also join the flexible sheets 26on both sides of the reinforcing insulative films 20 b to 20 d and 24 bto 24 d in the z-axis direction such that the reinforcing insulativefilms 20 b to 20 d and 24 b to 24 d are disposed therebetween. In theportion where the flexible sheets 26 are adjacent to each other withoutthe reinforcing insulative films 20 b to 20 d and 24 b to 24 d, thesurfaces of the flexible sheets 26 flow and the flexible sheets 26 arecoupled together. The via-hole conductors b1 to b3, b11, b12, b21 tob26, and b31 to b36, the wiring conductors 30 b, 30 c, 36 b, and 36 c,and the ground conductors 37 and 40 are electrically coupled to eachother. In this way, the circuit substrate 10 illustrated in FIG. 1 isobtained.

The circuit substrate 10 can have circuit characteristics in which theoccurrence of deviations can be reduced and can include relatively hardrigid regions R1 and R2 and relatively soft flexible region F1, asdescribed below. More specifically, in the circuit substrate describedin Japanese Unexamined Patent Application Publication No. 2006-339186, aredundant wiring pattern is disposed in the rigid portion. The redundantwiring pattern faces other wiring patterns and generates unnecessarystray capacitance. As a result, the circuit characteristics in thewiring substrate described in Japanese Unexamined Patent ApplicationPublication No. 2006-339186 deviate from a desired value.

In contrast, in the circuit substrate 10, the reinforcing insulativefilms 20 b to 20 d and 24 b to 24 d are disposed in the rigid regions R1and R2. Specifically, the reinforcing insulative film 20 b is disposedon the substrate-section sheet 27 b (in rigid region R1) so as to coverthe portion where the wiring conductors 30 b and the via-hole conductorsb2 and b21 to b23 are not disposed. The reinforcing insulative film 24 bis disposed on the substrate-section sheet 29 b (in the rigid region R2)so as to cover the portion where the wiring conductors 36 b and thevia-hole conductors b12 and b31 to b33 are not disposed. The reinforcinginsulative film 20 c is disposed on the substrate-section sheet 27 b (inthe rigid region R1) so as to cover the portion where the wiringconductors 30 c and the via-hole conductors b3 and b24 to b26 are notdisposed. The reinforcing insulative film 24 c is disposed on thesubstrate-section sheet 29 c (in the rigid region R2) so as to cover theportion where the wiring conductors 36 c and the via-hole conductors b34to b36 are not disposed. The reinforcing insulative film 20 d isdisposed on the substrate-section sheet 27 d (in the rigid region R1) soas to cover the portion where the ground conductor 37 is not disposed.The reinforcing insulative film 24 d is disposed on thesubstrate-section sheet 29 d (in the rigid region R2) so as to cover theportion where the ground conductor 40 is not disposed.

As described above, for the circuit substrate 10, the reinforcinginsulative films 20 b to 20 d and 24 b to 24 d are added in the rigidregions R1 and R2. Therefore, the rigid regions R1 and R2 are harderthan the flexible region F1 by the hardness corresponding to thereinforcing insulative films 20 b to 20 d and 24 b to 24 d. Thus, theexistence of the reinforcing insulative films 20 b to 20 d and 24 b to24 d makes the rigid regions R1 and R2 hard and eliminates the necessityto have an unnecessary conductive layer. Because each of the reinforcinginsulative films 20 b to 20 d and 24 b to 24 d is not a conductive layerbut an insulator layer, no stray capacitance occurs between thereinforcing insulative film and other wiring patterns. Accordingly, thecircuit substrate 10 can have circuit characteristics in which theoccurrence of deviations can be reduced, and in particular, when it isused in high-frequency ranges, a high-frequency characteristic, such asimpedance characteristic, is not prone to change. In addition, thecircuit substrate 10 can include the relatively hard rigid regions R1and R2 and the relatively soft flexible region F1.

In the circuit substrate 10, because the material of each of thereinforcing insulative films 20 b to 20 d and 24 b to 24 d is preferablyharder than that of each of the flexible sheets 26 (that is, has alarger Young's modulus), the rigid regions R1 and R2 are less prone todeformation.

In the circuit substrate 10, the reinforcing insulative films 20 b to 20d and 24 b to 24 d, which are preferably made of thermosetting resin,are disposed in the rigid regions R1 and R2. Therefore the occurrence ofplastic deformation caused by large warping of the rigid regions R1 andR2 in the circuit substrate 10 can be more reduced than that in the casewhere the rigidity is increased by the use of a metal material.

The circuit substrate 10 can also reduce separation of the flexiblesheets 26, as described below. More specifically, the thickness of thereinforcing insulative film 20 b is not larger than and substantiallythe same as that of the wiring conductor 30 b. The thickness of thereinforcing insulative film 20 c is not larger than and substantiallythe same as that of the wiring conductor 30 c. The thickness of thereinforcing insulative film 20 d is not larger than and substantiallythe same as that of the ground conductor 37. The thickness of thereinforcing insulative film 24 b is not larger than and substantiallythe same as that of the wiring conductor 36 b. The thickness of thereinforcing insulative film 24 c is not larger than and substantiallythe same as that of the wiring conductor 36 c. The thickness of thereinforcing insulative film 24 d is not larger than and substantiallythe same as that of the ground conductor 40. Accordingly, the existenceof the reinforcing insulative films 20 b to 20 d and 24 b to 24 d canreduce differences in height occurring on the front sides of theflexible sheets 26 caused by the wiring conductors 30 b, 30 c, 36 b, and36 c and the ground conductors 37 and 40. Therefore, gaps are preventedfrom being formed between the flexible sheets 26 when the flexiblesheets 26 are press-bonded. As a result, the flexible sheets 26 arefirmly attached together and are not prone to being separated. Becausethe reinforcing insulative films 20 b to 20 d and 24 b to 24 daccommodate differences in height caused by the thicknesses of thewiring conductors 30 b, 30 c, 36 b, and 36 c and the ground conductors37 and 40, the surface flatness in the circuit substrate 10, which is alamination of the flexible sheets 26, can be improved.

A circuit substrate 10 a according to a first variation of a preferredembodiment of the present invention is described below with reference tothe drawing. FIG. 5 is an exploded perspective view of the circuitsubstrate 10 a according to the first variation.

In the circuit substrate 10 a, the main body 11 includes semi-rigidregions SR1 and SR2, in addition to the rigid regions R1 and R2 and theflexible region F1. The semi-rigid region SR1 is disposed between therigid region R1 and the flexible region F1. The semi-rigid region SR2 isdisposed between the rigid region R2 and the flexible region F1. Therigid regions R1 and R2 are less deformable than the semi-rigid regionsSR1 and SR2. The flexible region F1 is more easily deformable than thesemi-rigid regions SR1 and SR2. The semi-rigid regions SR1 and SR2 areconfigured such that both ends of the flexible region F1 in the x-axisdirection are hard. Specifically, to form the semi-rigid regions SR1 andSR2, reinforcing insulative films 55 (55 b, 55 d) and 57 (57 b, 57 d)are disposed.

More specifically, the reinforcing insulative films 55 b and 57 b aredisposed so as to cover the portions where the ground lines 32 b, 33 b,and 34 b are not disposed in the semi-rigid regions SR1 and SR2,respectively, on the line-section sheet 31 b when seen in plan view fromthe z-axis direction. The reinforcing insulative films 55 d and 57 d aredisposed so as to cover the portions where the ground lines 32 d, 33 d,and 34 d are not disposed in the semi-rigid regions SR1 and SR2,respectively, on the line-section sheet 31 d when seen in plan view fromthe z-axis direction.

The above-described circuit substrate 10 a can reduce the occurrence ofbreakage in the main body 11 at the border between the flexible regionF1 and each of the rigid regions R1 and R2. More specifically, when thesemi-rigid regions SR1 and SR2 are not disposed between the rigidregions R1 and R2 and the flexible region F1, the hardness of the mainbody 11 significantly varies at the border between the flexible regionF1 and each of the rigid regions R1 and R2. If the line section(flexible region F1) is bent, stress concentrates on the border betweenthe flexible region F1 and each of the rigid regions R1 and R2. As aresult, the main body 11 may be curved and broken at the border betweenthe flexible region F1 and each of the rigid regions R1 and R2.

In contrast, the circuit substrate 10 a includes the semi-rigid regionsSR1 and SR2 between the rigid regions R1 and R2 and the flexible regionF1. Therefore, the hardness of the main body 11 varies in stages at theborder between the flexible region F1 and each of the rigid regions R1and R2. Therefore, if the line section 16 (flexible region F1) is bent,stress is distributed to the semi-rigid regions SR1 and SR2. As aresult, the occurrence of breakage in the main body 11 at the borderbetween the flexible region F1 and each of the rigid regions R1 and R2can be reduced.

A circuit substrate 10 b according to a second variation of a preferredembodiment of the present invention is described below with reference tothe drawing. FIG. 6 is an exploded perspective view of the circuitsubstrate 10 b according to the second variation.

In the circuit substrate 10 b, a coil L is incorporated in the substratesection 12 (rigid region R1). The coil L includes spiral coil conductors60 b and 60 c. Both ends of the coil L are connected to signal lines 64b and 64 c. The signal lines 64 b and 64 c extend in the line section 16along the x-axis direction. The signal lines 64 b and 64 c are connectedto wiring conductors 62 b and 62 c electrically coupled to the lands 35.The circuit substrate 10 b functions as a transmit/receive circuit forhigh-frequency signals by using the coil L as an antenna.

In the above-described circuit substrate 10 b, if the substrate section12, in which the coil L is disposed, is easily deformed, the inductancevalue of the coil L changes and the frequency characteristic of the coilL changes. Accordingly, the circuit substrate 10 b can reduce changes inthe frequency characteristic of the coil L by providing the substratesection 12 with the reinforcing insulative films 20 b and 20 c andthereby making the substrate section 12 be the rigid region R1, which isnot easily deformable.

A circuit substrate 10 c according to a third variation of a preferredembodiment of the present invention is described below with reference tothe drawing. FIG. 7 is a cross-sectional configuration view of thecircuit substrate 10 c according to the third variation.

In the circuit substrates 10, 10 a, and 10 b, the flexible sheets 26have the same shape when seen in plan view from the z-axis direction.That is, for the circuit substrates 10, 10 a, and 10 b, the number ofthe flexible sheets 26 is the same at any location.

In contrast, for the circuit substrate 10 c, the number of the flexiblesheets 26 in the flexible region F1 is smaller than the number of theflexible sheets 26 in the rigid regions R1 and R2. Thus, the thicknessin the flexible region F1 is smaller and the flexible region F1 issofter.

In manufacturing the circuit substrate 10 c, after the flexible sheets26 a to 26 d are press-bonded, the flexible sheets 26 a and 26 d in theflexible region F1 may be removed. Alternatively, the flexible sheets 26a and 26 d from which the portion in the flexible region F1 has beenremoved may be press-bonded.

In the circuit substrates 10 and 10 a to 10 c, each of the rigid regionsR1 and R2 indicates a region where electronic components, such as thechip components 50, are implemented, a region where the coil L isdisposed, and similar regions. The flexible region F1 indicates a regionwhere, even if it is deformed, circuit characteristics are not easilychanged and where the signal lines 42 c, 43 c, and 44 c and otherelements are disposed.

As described above, preferred embodiments of the present invention areuseful in a circuit substrate and a method of manufacturing the same. Inparticular, it is advantageous in that a circuit substrate that hascircuit characteristics in which the occurrence of deviations can bereduced and that includes a relatively hard region and a relatively softregion is obtainable.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A circuit substrate comprising: a main body including a stack of aplurality of first insulator layers made of a flexible material, themain body including a rigid region and a flexible region, the flexibleregion being more easily deformable than the rigid region; a conductivelayer disposed in the main body and defining circuitry; and a secondinsulator layer disposed so as to cover at least a portion of an areawhere the conductive layer is not disposed in the rigid region on atleast one of the first insulator layers when seen in plan view from astacking direction.
 2. The circuit substrate according to claim 1,wherein the second insulator layer is made of a material that is harderthan a material of the first insulator layers.
 3. The circuit substrateaccording to claim 1, wherein the second insulator layer is disposed inthe main body, and the second insulator layer has a thickness equal toor less than a thickness of the conductive layer in the stackingdirection.
 4. The circuit substrate according to claim 1, wherein theconductive layer extends across a border between the rigid region andthe flexible region.
 5. The circuit substrate according to claim 1,wherein the main body further includes a semi-rigid region between therigid region and the flexible region, the rigid region is lessdeformable than the semi-rigid region, the flexible region is moreeasily deformable than the semi-rigid region, and the second insulatorlayer is disposed so as to cover a portion where the conductive layer isnot disposed in the semi-rigid region on at least one of the firstinsulator layers when seen in plan view from the stacking direction. 6.A method of manufacturing a circuit substrate comprising: a step ofpreparing a plurality of first insulator layers on which circuitry madeof a conductive layer is formed, the plurality of first insulator layersbeing made of a flexible material; a step of forming a second insulatorlayer so as to cover at least a portion of an area where the conductivelayer is not disposed on at least one of the first insulator layers whenseen in plan view from a stacking direction of the first insulatorlayers; and a step of stacking and press-bonding the plurality of firstinsulator layers.